The invention concerns an inductance device. It concerns particularly short short wave devices, for high power transmission, for example 100 kw and above, such as are found for example in a radio transmitter.
An example of a short wave radio transmitter operating between 6 and 30 MHz notably includes an adaptor unit which incorporates an inductance device in the form of a coil made up of several turns side by side along an axis.
The design of an inductance device of this type operating over a wide frequency band is always a delicate problem. Indeed, this circuit only conserves its pure inductance properties in a restricted frequency band owing to parasite capacities, appearing as the frequency increases, which cause a considerable decrease in the resonance frequency of a circuit formed in this way. It is only below this resonance frequency, then, that the circuit has inductance properties.
This is particularly true in the present example where a circuit of large dimensions is necessary owing to the large currents involved and the high voltages which appear at the inductance terminals, etc., dimensions which are not negligible in relation to the wavelengths of the frequencies used.
Moreover, in the case where this type of inductance coil is adjustable by short circuiting a section of the coil of variable length, a dead section formed by this part of the coil lies in the magnetic field along the axis of the coil. Induced currents are thus produced in this dead section and modify the performance of the inductance assembly.
For these various reasons, it is at present almost impossible to anticipate -- either by measurement or calculation -- the behavior of such an inductance device in a frequency ratio which can range from about 4.3 to 8.6.